(a) Field of the Invention
The present invention relates to a display device using a mixture of a dichroic dye serving as a guest material and a liquid crystal material serving as a host material to effect a display of a colored image and, more particularly, it pertains to such guest-host type liquid crystal display device which does not require the use of external polarizers.
(b) Description of the Prior Art
There have been proposed heretofore many display devices of the so-called guest-host type which use a mixture of a pleochroic dye as a guest material and a liquid crystal material as a host material in a display cell for the purpose of effecting a display of a colored image. Most of them employ a polarizer for polarizing light rays which impinge onto the cell or for analyzing the plane of light rays leaving the cell. However, there has been also proposed a guest-host type display device which does not require the use of such external polarizers. FIG. 6 shows a cross-sectional sketch showing essential aspects of such display device of the so-called White & Taylor type (see, e.g. J. Appl. Phys. Vol. 45, No. 11, November 1974). As shown in FIG. 6, such display device comprises a mixture of a dichroic dye and a liquid crystal material placed between two parallel transparent substrates a and b such as glass plates and sealed with spacers e and f which are placed at the peripheral portions of these substrates. The substrates a and b have transparent electrodes c and d, respectively, which are formed on their inner surfaces so as to adhere thereto. Liquid crystal materials to be used in the White & Taylor type display device are those which can assume a cholesteric orientation when no voltage is applied between the electrodes. That is, for example, as shown in a pattern denoted by "g" in FIG. 6, the molecules of the liquid crystal are arranged in a direction substantially parallel to the plane of the substrate, but in a helical configuration as a whole. Upon application of a voltage between the electrodes c and d, the liquid crystal molecules are caused to be rearranged to assume the so-called homeotropic nematic state where the longitudinal axis of the molecules are oriented in a direction perpendicular to the plane of the substrate, i.e., in a direction parallel to the direction of the applied electric field. This homeotropic nematic orientation is illustrated in FIG. 6 as denoted by "g'." Thus, when no voltage is applied, it should be noted that, among the incident light rays, those having a wavelength in the range of the absorption band characteristic to the dichroic dye are absorbed by the dye, so that the surface of the display cell is viewed to be in the color of the dye. When a voltage is applied between the electrodes c and d, only the liquid crystal molecules existing therebetween are rearranged to assume the aforementioned homeotropic nematic state. Consequently, the light absorption caused by the dye is minimized in the region between the electrodes, so that the display cell can produce a display of a substantially clear image corresponding to the pattern of the electrodes against the colored background.
Although the White & Taylor type display cell can be operated without using either an external polarizer nor an external analyzer as explained above, it has the disadvantage that it requires a relatively high voltage to perform a display of an image having a satisfactory contrast against the colored background. The contrast may be enhanced by decreasing a helical pitch of the cholesteric orientation. However, such decrease in the helical pitch is inevitably accompanied by an increase in the threshold value of voltage which is required for the rearrangement of the liquid crystal molecules into homeotropic nematic orientation. In order to achieve an optimum contrast, a voltage as high as several tens of volts is generally required. Conversely, in order to lower the threshold value of voltage to a practically acceptable level of about ten volts or less, a significant reduction of the contrast cannot be avoided.